Angle at centre twice angle at circumference This is a prototype of JavaSketchpad, a World-Wide-Web component of The Geometer's Sketchpad. Copyright ©1990-1998 by Key Curriculum Press, Inc. All rights reserved. Portions of this work were funded by the National Science Foundation (awards DMI 9561674 & 9623018). Sorry, this page requires a Java-compatible web browser. Drag points A , P and B, and observe the relationship between the above angles. Can you make use of a property about triangles (that you learned in F.2) to prove the relationship between the above angles?

Angle in semi-circle A, B, C are points on a circle such that AB is a diameter. Drag B and C and guess the measure of the angle ACB. Check your guess by clicking the button "Angle ACB". Can you use the above theorem to prove your conjecture?

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Angles in the same segment Drag C along the circle. What can you say about the measure of the angle ACB? Click the button "Question" to show the angle ADB. Can you prove the relationship between the angles ACB and ADB?

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